102 LATENT HEAT. 



exists sufficient heat to raise several square inches of 

 metal to glowing redness. By the compression of 

 atmospheric air this may be shown, and with a small 

 condensing syringe a sufficient quantity of heat may be 

 set free to fire the Boletus igniarius, which, impregnated 

 with nitre, is known as amadou. We are acquainted 

 with various sources from which heat may be developed 

 for artificial purposes : the flint-and-steel is an example 

 of the production of heat by mechanical force, and the 

 modern lucifer-match, of the combined action of friction 

 and chemical affinity. These of themselves would admit 

 of a lengthened discourse ; but it is necessary that we 

 carefully examine some of the less familiar phenomena of 

 lieat under the influences of changes of chemical condition. 

 If spirits of wine and water are mixed together, a 

 considerable degree of heat is given out, and by mixing 

 sulphuric acid and water, an infinitely larger quantity. 

 If sulphuric acid (oil of vitriol) and spirit of wine, or 

 nitric acid (aquafortis) and spirits of turpentine, at com- 

 mon temperatures, be suddenly mixed, so much heat is- 

 set free as to ignite the spirit. In each of these in- 

 stances there is a condensation of the fluid. In nearly 

 all cases of solution, cold is produced by the absorption 

 of the heat necessary to sustain the salt in a liquid 

 form ; but when potash dissolves in water, heat is given 

 out, which is a fact we cannot yet explain. If potassium 

 is placed on water, it seizes the oxygen of the water 

 and sets fire to the hydrogen gas liberated by the heat 

 produced in the change of form. Antimony and many 

 other metals thrown into chlorine gas ignite and bum 

 with brilliancy: the same phenomenon takes place in 

 the vapours of iodine or bromine. Many chemical com- 

 binations, as the chloride of potassium and sulphur ex- 

 plode with a blow ; whilst the slightest friction occasions 

 the detonation of the fulminating salts of silver, mercury, 

 and gold. Compounds of nitrogen and chlorine, or 

 iodine, are still more delicately combined the former 



